How to centre align a circle to be above another circle in other layer?
In Object Mode, the action Selection to cursor will move the object such that its origin is at the cursor. For your object, it's origin is not at its center, so it does not line up to the eye.
First, go into edit mode on your pupil and use Set Origin &gt; Origin to Geometry. This will bring your origin to the center of the pupil, so when you then run Selection to Cursor, it will be properly centered.